The Source of Europe's Mild Climate

If you grow up in England, as I did, a few items of unquestioned
wisdom are passed down to you from the preceding generation. Along
with stories of a plucky island race with a glorious past and the
benefits of drinking unbelievable quantities of milky tea, you will
be told that England is blessed with its pleasant climate courtesy
of the Gulf Stream, that huge current of warm water that flows
northeast across the Atlantic from its source in the Gulf of Mexico.
That the Gulf Stream is responsible for Europe's mild winters is
widely known and accepted, but, as I will show, it is nothing more
than the earth-science equivalent of an urban legend.

This is not to say that there is no climatological mystery to be
explained. The countries of northern Europe do indeed have curiously
mild climates, a phenomenon I didn't really appreciate until I moved
from Liverpool to New York. I arrived in the Big Apple just before a
late-summer heat wave, at a time when the temperature soared to
around 35 degrees Celsius. I had never endured such blistering
temperatures. And just a few months later I was awestruck by the
sensation of my nostrils freezing when I went outside. Nothing like
that happens in England, where the average January is 15 to 20
degrees warmer than what prevails at the same latitude in eastern
North America. So what keeps my former home so balmy in the winter?
And why do so many people credit the Gulf Stream?

Like many other myths, this one rests on a strand of truth. The Gulf
Stream carries with it considerable heat when it flows out from the
Gulf of Mexico and then north along the East Coast before departing
U.S. waters at Cape Hatteras and heading northeast toward Europe.
All along the way, it warms the overlying atmosphere. In the seas
between Norway and Newfoundland, the current has lost so much of its
heat, and the water has become so salty (through evaporation), that
it is dense enough to sink. The return flow occurs at the bottom of
the North Atlantic, also along the eastern flank of North America.
This overturning is frequently referred to as the North Atlantic
thermohaline circulation, or simply the "Atlantic
conveyor." It is part of the global pattern of ocean
circulation, which is driven by winds and the exchange of heat and
water vapor at the sea surface.

The Gulf Stream indeed contributes to Europe's warmth, but it is
wrong to conflate the climate difference across the North Atlantic
with the northward flow of warm water in the Gulf Stream. This
erroneous logic leads to such statements as (from TheTimes of London): "The British Isles lie on the same
latitude as Labrador on the East Coast of Canada, and are protected
from a similarly icy climate by the Atlantic conveyor belt."
Such claims are absolutely wrong.

The statements scientists make about Atlantic thermohaline
circulation typically read more like this one from my Columbia
University colleague, Wallace S. Broecker:

One of the major elements of today's ocean system is a
conveyor-like circulation that delivers an enormous amount of
tropical heat to the northern Atlantic. During winter, this heat is
released to the overlying eastward air masses, thereby greatly
ameliorating winter temperatures in northern Europe.

This assertion has the benefit of being both correct and misleading.
Because it does not specify what European climate is ameliorated
relative to (the climate of eastern North America?), it leaves
unchallenged the incorrect version expounded in the popular
media—thus contributing to the erroneous beliefs of millions.

The idea that the Gulf Stream is responsible for Europe's mild
winters seems to have originated with Matthew Fontaine Maury, an
American naval officer who in 1855 published The Physical
Geography of the Sea, which is often considered the first
textbook of physical oceanography. The book was a huge success, went
through many printings and was translated into three languages. The
role of the Gulf Stream in shaping climate is a recurring theme in
Maury's book. For example, he stated:

One of the benign offices of the Gulf Stream is to convey
heat from the Gulf of Mexico, where otherwise it would be excessive,
and to disperse it in regions beyond the Atlantic for the
amelioration of the climates of the British Isles and of all Western
Europe.

According to Maury, if this transport of heat did not take place,
"… the soft climates of both France and England would be
as that of Labrador, severe in the extreme, and ice bound."
Despite the differences in language and style, the modern statements
clearly owe their provenance to this 1855 treatise.

Maury thought that God set the ocean up to work this way apparently
as part of His design to keep Europe warm (for unspecified reasons).
But holding such religious beliefs did not stop Maury from also
providing a scientific explanation for the Gulf Stream. His idea was
that it was the oceanic equivalent of what in the atmosphere is
known as a Hadley cell, a convection cell wherein warm air
flows upward and poleward, and cold material flows downward and
equatorward. In the ocean, heated surface waters take a
northeastward route, in Maury's view, because of the need to
conserve angular momentum as they move north and, hence, closer to
the axis of the Earth's rotation. Maury did not recognize that winds
drive ocean currents. And it was not until a century later that a
valid explanation of the Gulf Stream emerged: In the jargon of
oceanographers, it is a westward-intensified boundary current within
a subtropical gyre (a large circular current system) driven
by the trade winds, which blow from east to west in the tropics, and
mid-latitude westerlies, which move in the opposite direction.